1. Field of the Invention
The present invention relates to a magnetic recording and reproducing apparatus such as a hard disk drive.
2. Description of the Related Art
Conventionally, a hard-disk drive adopts a system wherein servo information for controlling positioning of a magnetic head is recorded on a recording medium for dedicated use, which is different from a recording medium (disk) on which data is read/written by a user. According to this system, servo information can be continuously obtained, but a recording medium for dedicated recording servo information must be provided. As a result, the recording density of the entire hard disk is low.
In order to meet the demand for a higher recording density, a system has been proposed wherein servo information is recorded in advance in the same recording medium as is employed for storing read/write data by the user, A typical system is a sector servo system. According to this system, a plurality of tracks are provided concentrically in each recording medium. Each track is divided into a plurality of sectors. Each sector includes a data storage area in/from which data is written/read by the user, a servo storage area in which servo information for positioning control of a magnetic head, and an area in which a track number and a sector number in each track are recorded. According to the sector servo system, the servo information is recorded intermittently in the recording medium.
In the system wherein servo information is recorded in the dedicated recording medium, a displacement in position occurs between a servo head (a head for reading servo information) and a data head (a head for reading/writing data treated by the user) due to a difference in thermal expansion of mechanical parts. On the other hand, no such displacement occurs in the sector servo system, but it is necessary to record servo information in all recording mediums. In addition, servo information cannot be obtained continuously. In particular, since servo information is recorded in advance in all recording mediums before the products are shipped, the time for production is adversely affected in the case of mass-producing recording mediums.
FIG. 1 shows schematically the structure of the recording medium. When the hard-disk drive is not activated, as shown in FIG. 1, that is, the power is off, a magnetic head 11 is situated in contact with a contact start and stop (CSS) zone 13 provided in an inner annular portion of the recording medium 12. The CSS zone 13 is provided on the recording medium 12, separately from a data zone 14. Information is not stored commonly in the CSS zone 13.
When the power is turned on and the rotation speed of the recording medium 12 reaches a predetermined value, the magnetic head 11 floats from the recording medium 12 and thereafter the magnetic head 11 starts to move towards the track 0 (zero) provided in the radially outermost portion. This is called "first seek." In particular, in the first seek after servo information is recorded by a servo writer, each track of the recording medium 12 is in the initial state, as shown in FIG. 2. That is, since each track has not been "manufacture-formated," information other than servo information cannot be read out from the servo area of each track. The term "manufacture format" in this context means a physical format produced by a pre-shipment test on a recording medium on which only servo information is recorded, and differs from an ordinary format obtained by the user's formatting. Although it is possible to record not only the servo information but also other information on the recording medium by using the servo writer, recording of information by the servo writer consumes a considerable time. The manufacture time can be made shorter when only servo information is recorded on the recording medium by using the servo writer resulting in higher manufacture efficiency.
Since no information is recorded in the CSS zone 13, as stated above, an automatic gain control (AGC) gain for controlling the level of an output signal of the magnetic head 11 is set at a maximum value while the magnetic head 11 moves over the CSS zone 13. When the magnetic head 11 moves over the data zone 14 and starts to read out servo information recorded on the servo area of each track, the AGC gain is stabilized (see FIG. 2).
However, in the first seek, it is highly possible that the magnetic head 11 overruns before the AGC gain is stabilized. This is because servo information cannot be read exactly in the case where the AGC gain is not stable and the seek control of the magnetic head is disabled. If the magnetic head 11 overruns, data read/write is disabled and, in addition, the magnetic head 11 moves as if to go beyond the radially outermost portion of the recording medium 12, as a result of which the magnetic head 11 and recording medium 12 may be damaged.
Under the situation, there is a demand for a magnetic recording and reproducing apparatus capable of stabilizing the AGC gain at the time of the first seek, without lowering the manufacture efficiency.